1. Field of the Invention
The present invention relates to an image printing apparatus, a method of controlling the same, and a printing apparatus.
2. Description of Related Art
In recent years, color printers capable of printing color images on printing media are popularly used. However, when a monochromatic image of characters or the like is to be printed by a color printer, the printing speed decreases, as will be described later. As a solution to this problem, a color printer which has both a color ink head for printing a color image and a black ink head for printing a binary image and selectively uses them as needed to allow printing both a color image and a monochromatic image on one printing medium has been used.
In the medical field where radiographs or CT/MRI images are used, monochromatic images are still used in many cases. This is because the density resolution of human eyes is high. In the medical field where high density resolution is required, a larger quantity of information can be visually recognized from a monochromatic image than from a color image.
Additionally, as is known, the density resolution of human eyes is higher for a transparent printing medium than for a reflective printing medium. It is generally said that human eyes have density resolution of about 8 bits for a color image and 10 to 11 bits for a monochromatic transmission image.
A radiograph or CT/MRI image is printed on a transparent printing medium and provided as a medical image. A doctor reads the image at the critical density resolution of human eyes, thereby obtaining a diagnostic result. Although images are used in the medical field, ultrasonic diagnosis, nuclear medical apparatuses, endoscopes, retinal cameras, and pathological microscopes often use color images for the purpose of obtaining color vital information or expressing functional vital information such as blood stream states.
Conventionally, printing apparatuses for printing color images and those for printing monochromatic high-gradation images are independently prepared and selectively used. For this reason, a color image and a monochromatic high-gradation image cannot be simultaneously printed on one printing medium. Management of printed images is also cumbersome.
There are also color image printing apparatuses capable of printing monochromatic images. However, they are poorer in gradation expression than printing apparatuses exclusively used to print monochromatic images. In addition, a printing medium for color image printing and that for monochromatic image printing need be selectively used depending on applications.
An example of such an apparatus is a sublimation thermal transfer printer. In this apparatus, three ink ribbons (dyes) of Y, M, and C or R, G, and B are prepared. An ink ribbon overlapping a printing medium is partially heated by a thermal head to transfer the dye of the ink ribbon to the medium, thereby forming an image. When the same process is repeated three times for the respective ink ribbons, a color image can be formed. To print a monochromatic image by this scheme, the three different color inks are uniformly overlaid. In this scheme, however, a monochromatic image is expressed by overlaying three colors, and it is difficult to express a neutral monochrome without any color appearance. In addition, a sufficient monochromatic density (e.g., OD3) cannot be expressed particularly for a transparent medium.
For this reason, when a neutral monochromatic density or sufficiently high monochromatic density is required, a heat-sensitive medium for printing monochromatic images is independently prepared and partially heated by the thermal head. By blackening the heated portion, an image is obtained. That is, a medium for color images is exchanged with the medium for monochromatic images, and the ink ribbons are detached as needed.
As another example, there is an ink-jet printer. In this scheme, three different color inks: Y, M, and C or R, G, and B are prepared and overlaid to express a color image. In this case as well, a monochromatic image can be expressed by uniformly overlaying the three colors. However, a neutral monochrome without any color appearance can hardly be expressed because the three colors are overlaid, as in the sublimation thermal transfer printer. To express a sufficient monochromatic density (e.g., OD3) particularly for a transparent medium, inks must be overlaid on the same pixel. However, the ink absorption amount of a medium is limited, so a sufficient monochromatic density cannot be expressed. More specifically, to realize the gradation of an image or increase the density, inks are overlaid on the same pixel. However, the ink absorption amount of a printing medium is limited. If inks are overlaid beyond this limitation, inks overflow to blur the image.